- Email: [email protected]
Optical and structural properties of SnO2 films grown by a low-cost CVD technique
May 2002
Materials Letters 54 (2002) 158 – 163 www.elsevier.com/locate/matlet
Optical and structural properties of SnO2 films grown by a low-cost CVD technique P. Rajaram a,*, Y.C. Goswami a, S. Rajagopalan b, V.K. Gupta a a
School of Studies in Physics, Jiwaji University, Gwalior 474 011 (MP), India b Materials Science Division, IGCAR, Kalpakkam, India
Received 15 March 2001; received in revised form 30 July 2001; accepted 4 August 2001
Abstract Highly conducting and transparent SnO2 thin films have been grown by a simple and low-cost CVD technique. The films ˚ thick grown on quartz and glass slides have both good conductivity and high transparency. Undoped SnO2 films about 2500 A have a sheet resistance below 100 V/5 and transmission between 94% and 99.9% in most of the visible spectrum. Thicker films (around 500 nm to 1 Am) have a sheet resistance as low as 10 V/5 with a transmission coefficient of 70% in the visible spectrum. XRD studies show that as the growth temperature is increased from 300 toward 400 jC, the diffraction peaks become sharper, indicating an improved crystallinity. Films grown at 400 jC have the best crystallinity with a very sharp and highly intense major peak. Secondary ion mass spectrometry (SIMS) depth profiles show that the films have a uniform composition along the depth. D 2002 Elsevier Science B.V. All rights reserved. Keywords: Thin films; Tin oxide; Transparent conductors; CVD
1. Introduction Thin films of SnO2 are being used in a wide range of applications, e.g., electrodes in electroluminiscent displays, imaging devices, protective coatings, antireflection coatings, gas and chemical sensors, transducers, applications based on transparent conductors including heating windows of aircraft and cars and incandescent lamps, solar cells and other opto-electronic devices [1– 6]. Various methods including rf sputtering, electron beam evaporation, CVD, spray pyrolysis, photochemical vapor deposition and the *
Corresponding author. E-mail address: [email protected] (P. Rajaram).
sol – gel method have been used to deposit these films [4,7 –11]. Various dopants like antimony, boron, fluorine and chlorine have been used to improve the conductivity of the films [12 –15]. In this work, we present a simple and low-cost technique, which has been used to grow highly conducting and transparent layers of undoped SnO2.
2. Experimental details A.R. Grade Stannous chloride (SnCl2 2H2O) obtained from Ranbaxy Lab. (India) was used as the starting material throughout this work although, in some cases, stannic chloride (SnCl4 5H2O) was also
0167-577X/02/$ - see front matter D 2002 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 7 - 5 7 7 X ( 0 1 ) 0 0 5 5 5 - 9
P. Rajaram et al. / Materials Letters 54 (2002) 158–163
159
Fig. 1. Experimental setup for growth of SnO2 films.
used for comparison. The CVD setup is shown in Fig. 1 and basically consists of an oxygen gas cylinder, which is connected through a flowmeter to a glass bubbler, which contains the amorphous SnCl2. A second glass tube connects the bubbler to a funnel, which is placed upside down on a hot plate. The bubbler containing the SnCl2 is heated to almost the melting point of SnCl2 and oxygen is passed through it. The vapors of SnCl2 are thus carried through the tube and funnel on to the substrates, which are kept on the hot plate. Although a similar CVD setup has been used by Kojima et al. [16] in which the substrates are heated on a hot plate, our system is new in the sense that it is much simpler, employing a single oxygen gas inlet, thus requiring no additional mixing stage. The SnCl2 vapors produced by heating the bubbler are directly carried to the substrates by the oxygen gas. We have also observed that the use of the funnel to form the reaction chamber leads to a uniform film growth even at very low flow rates. A similar setup was also used by Yadava et al. [17] for the growth of SnO2 films. Their setup used a jet nozzle to direct the SnCl2 vapours onto substrates instead of the funnel used in this work. Their deposition chamber was also somewhat more elaborate, consisting of a stainless steel and glass enclosure over a furnace.
Glass and quartz slides cleaned with chromic acid, liquid soap and distilled water followed by ultrasonic cleaning were used as the substrates. Thin films were grown at various substrate temperatures ranging from 300 to 500 jC, the monitoring of temperature being done with the help of a Chromel –Alumel thermocouple. Optical Transmission spectra of the SnO2 films on quartz in the wavelength range 200 –900 nm were obtained on a Shimadzu UV – VIS Spectrophotometer. a (the absorption coefficient) and (a ht)2 vs. ht plots were obtained on a computer to evaluate the band gap of the grown SnO2 layers. The thickness of the films was obtained using the spectroscopic method. X ray diffraction was performed using Cu K a radiation on a Rigaku Rotaflex Diffractometer. Secondary ion mass spectra (SIMS) depth profiles were obtained on a CAMECA IMS5F instrument.
3. Results and discussion 3.1. Structural properties The X-ray diffractograms of SnO2 films grown (using SnCl2) at substrate temperatures of 300, 350, 400 and 450 jC are shown in Fig. 2(a – d). Films
160 P. Rajaram et al. / Materials Letters 54 (2002) 158–163
Fig. 2. X-ray diffractograms for films grown at substrate temperatures (a) 300 jC, (b) 350 jC, (c) 400 jC, (d) 450 jC.
P. Rajaram et al. / Materials Letters 54 (2002) 158–163
grown at and below 300 jC were amorphous, whereas those grown at 350 jC had a better crystallinity, which improved further as the temperature was increased from 350 to 400 jC. The best crystallinity was obtained at 400 jC, a substrate temperature in which a highly oriented (200) growth occurs. Interestingly, as the substrate temperature is further increased from 400 to 450 jC, the preferred orientation changes from (200) to (211). The crystal structure in all the cases was, however, the tetragonal cassiterate structure. SnO2 films, prepared by the same technique using SnCl4 instead of SnCl2, showed similar results. These two kinds of films, however, differed in the stability of the electrical properties. Although both types of films had high transmission and good conductivity, the electrical properties of the films grown from SnCl4 were unstable to postdeposition annealing. Heat treatments even at temperatures as low as 200 jC (and higher) led to a change in the conductivity of the
161
films. The electrical parameters of the films grown from SnCl2 were, on the other hand, highly stable and did not deteriorate during postdeposition annealing even at temperatures as high as 450 jC. This was particularly important when the SnO2 layers were used in our lab for CdS-based solar cells. Since the CdS was deposited on SnO2 at temperatures around 350 jC, the fact that the sheet resistance of SnO2 did not change during the deposition of CdS was a plus point. Structurally too, the films grown from SnCl2 had a superior crystallinity. 3.2. SIMS analysis SIMS depth profiles of the SnO2 films were obtained to study the uniformity of growth of the layers. Fig. 3 shows the depth profile of the 120Sn and 16 O signals obtained from a typical SnO2 sample on which another semiconductor CuInSe2 was grown. The almost-flat profile along the depth confirms that the composition of the film is uniform along the depth of the film. Depth profiles of heterojunctions like SnO2 /CdS/CuInS2 were also studied and these too confirmed the uniformity of growth of SnO2.
4. Optical and electrical properties
Fig. 3. SIMS depth profile for a CVD grown SnO2 film.
Fig. 4 shows the transmission spectrum of a SnO2 film grown on quartz at 400 jC. It shows that the transmission coefficient is between 94% and 99.9% in the whole visible range. This sample has a sheet resistance slightly less then 100 V/5 and a thickness ˚ . Fig. 5 shows the plot of (a ht)2 vs. close to 2500 A ht whose linear intercept gives a direct band gap of 4 eV, which is close to the values reported in Ref. [9]. ˚ to 1 Layers of higher thickness (between 5000 A Am) have a much lower sheet resistance at the expense of a slight fall in the transmission. Films having a sheet resistance of 10 V/5 with a transmission of about 70% have been routinely obtained with very good repeatability. Further improvement in transmission along with low sheet resistance will require doping of the films with dopants like Fluorine, and such attempts are going on in our lab. The resistivity of the SnO2 films decreased as the growth temperature increased from 300 to 400 jC. Films grown between
162
P. Rajaram et al. / Materials Letters 54 (2002) 158–163
Fig. 4. Transmission curve for SnO2 film grows on quartz.
400 and 450 jC had the lowest resistivity and highest transmission. The best films had a figure of merit defined by T 10/ R 5 [18] around 3.5 10 3. The SnO2 films grown at 400 jC had the lowest sheet resistance, ranging from 10 to 100 V /5, depending on the thickness of the layers. Films with lower sheet resistance 3 – 6 V /5 could be grown by increasing the thickness of the deposit; however, the optical transmission fell down to about 50%.
paper can be used to obtain highly conducting and highly transparent films of SnO2. The method gives fairly uniform SnO2 films and has very good repeatability. Films grown at 400 jC had the best crystallinity with a highly oriented growth. The electrical resistivity of the films grown using SnCl2 are highly stable to postdeposition annealing.
Acknowledgements 5. Conclusions In conclusion, we have demonstrated that the simple and low-cost CVD technique described in this
We are grateful to the Department of Science and Technology (DST), New Delhi, India, for supporting this work. We are thankful to IUC, Indore and IGCAR, Kalpakkam for providing us with the X-ray
P. Rajaram et al. / Materials Letters 54 (2002) 158–163
163
References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] Fig. 5. (a ht)2 vs. ht for SnO2 film (on quartz).
[15]
diffraction and SIMS facilities, respectively. We are also thankful to Dr. S.K. Gupta, S.O.S. in Chemistry, Jiwaji University for providing the spectrophotometer facility.
[16] [17] [18]
J.C. Anderson, J. Vac. Sci. Technol., A 4 (3) (1986) 610. K.D. Wise, J. Vac. Sci. Technol., A 4 (3) (1986) 617. S.M. Budd, Thin Solid Films 77 (1986) 13. A.K. Saxena, S.P. Singh, O.P. Agnihotri, Solar Cells 19 (1986 – 1987) 163. K.L. Chopra, S. Major, D.K. Pandya, Thin Solid Films 102 (1983) 1. H. Nishimura, Bus. Jpn. 9 (1991) 51. P. Smith, R. Singh, J. Dubow, Appl. Phys. Lett. 51 (4) (1980) 2164. K.B. Sundram, G.K. Bhagavat, Thin Solid Films 78 (1981) 35. J.C. Manifacier, M. De Murcia, J.P. Fillard, E. Vacario, Thin Solid Films 41 (1977) 127. G. Gasparro, J. Putz, Sol. Energy Mater. Sol. Cells 38 (1998) 287. S. Tamura, T. Ishida, H. Mihara, O. Tabata, T. Tatsuta, Thin Solid Films 343 – 344 (1999) 142. T. Arai, J. Phys. Soc. Jpn. 15 (1960) 916. S.A. Nasser, Thin Solid Films 342 (1999) 47. A. Bhardwaj, B.K. Gupta, A. Raza, A.K. Sharma, O.P. Agnihotri, Solar Cells 5 (1981 – 1982) 39. A. Ortiz, S. Lopez, C. Falcony, M.H. Farias, G. Soto, L. CotaRaiza, Sol. Energy Mater. 20 (1990) 381. M. Kojima, H. Kato, A. Imai, J. Appl. Phys. 64 (4) (1988) 1902. Y.P. Yadava, G. Denicolo, A.C. Arias, L.S. Roman, L.A. Hummelgen, Mater. Chem. Phys. 48 (1997) 263 – 267. G. Haacke, J. Appl. Phys. 47 (1976) 4086.
Materials Letters 54 (2002) 158 – 163 www.elsevier.com/locate/matlet
Optical and structural properties of SnO2 films grown by a low-cost CVD technique P. Rajaram a,*, Y.C. Goswami a, S. Rajagopalan b, V.K. Gupta a a
School of Studies in Physics, Jiwaji University, Gwalior 474 011 (MP), India b Materials Science Division, IGCAR, Kalpakkam, India
Received 15 March 2001; received in revised form 30 July 2001; accepted 4 August 2001
Abstract Highly conducting and transparent SnO2 thin films have been grown by a simple and low-cost CVD technique. The films ˚ thick grown on quartz and glass slides have both good conductivity and high transparency. Undoped SnO2 films about 2500 A have a sheet resistance below 100 V/5 and transmission between 94% and 99.9% in most of the visible spectrum. Thicker films (around 500 nm to 1 Am) have a sheet resistance as low as 10 V/5 with a transmission coefficient of 70% in the visible spectrum. XRD studies show that as the growth temperature is increased from 300 toward 400 jC, the diffraction peaks become sharper, indicating an improved crystallinity. Films grown at 400 jC have the best crystallinity with a very sharp and highly intense major peak. Secondary ion mass spectrometry (SIMS) depth profiles show that the films have a uniform composition along the depth. D 2002 Elsevier Science B.V. All rights reserved. Keywords: Thin films; Tin oxide; Transparent conductors; CVD
1. Introduction Thin films of SnO2 are being used in a wide range of applications, e.g., electrodes in electroluminiscent displays, imaging devices, protective coatings, antireflection coatings, gas and chemical sensors, transducers, applications based on transparent conductors including heating windows of aircraft and cars and incandescent lamps, solar cells and other opto-electronic devices [1– 6]. Various methods including rf sputtering, electron beam evaporation, CVD, spray pyrolysis, photochemical vapor deposition and the *
Corresponding author. E-mail address: [email protected] (P. Rajaram).
sol – gel method have been used to deposit these films [4,7 –11]. Various dopants like antimony, boron, fluorine and chlorine have been used to improve the conductivity of the films [12 –15]. In this work, we present a simple and low-cost technique, which has been used to grow highly conducting and transparent layers of undoped SnO2.
2. Experimental details A.R. Grade Stannous chloride (SnCl2 2H2O) obtained from Ranbaxy Lab. (India) was used as the starting material throughout this work although, in some cases, stannic chloride (SnCl4 5H2O) was also
0167-577X/02/$ - see front matter D 2002 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 7 - 5 7 7 X ( 0 1 ) 0 0 5 5 5 - 9
P. Rajaram et al. / Materials Letters 54 (2002) 158–163
159
Fig. 1. Experimental setup for growth of SnO2 films.
used for comparison. The CVD setup is shown in Fig. 1 and basically consists of an oxygen gas cylinder, which is connected through a flowmeter to a glass bubbler, which contains the amorphous SnCl2. A second glass tube connects the bubbler to a funnel, which is placed upside down on a hot plate. The bubbler containing the SnCl2 is heated to almost the melting point of SnCl2 and oxygen is passed through it. The vapors of SnCl2 are thus carried through the tube and funnel on to the substrates, which are kept on the hot plate. Although a similar CVD setup has been used by Kojima et al. [16] in which the substrates are heated on a hot plate, our system is new in the sense that it is much simpler, employing a single oxygen gas inlet, thus requiring no additional mixing stage. The SnCl2 vapors produced by heating the bubbler are directly carried to the substrates by the oxygen gas. We have also observed that the use of the funnel to form the reaction chamber leads to a uniform film growth even at very low flow rates. A similar setup was also used by Yadava et al. [17] for the growth of SnO2 films. Their setup used a jet nozzle to direct the SnCl2 vapours onto substrates instead of the funnel used in this work. Their deposition chamber was also somewhat more elaborate, consisting of a stainless steel and glass enclosure over a furnace.
Glass and quartz slides cleaned with chromic acid, liquid soap and distilled water followed by ultrasonic cleaning were used as the substrates. Thin films were grown at various substrate temperatures ranging from 300 to 500 jC, the monitoring of temperature being done with the help of a Chromel –Alumel thermocouple. Optical Transmission spectra of the SnO2 films on quartz in the wavelength range 200 –900 nm were obtained on a Shimadzu UV – VIS Spectrophotometer. a (the absorption coefficient) and (a ht)2 vs. ht plots were obtained on a computer to evaluate the band gap of the grown SnO2 layers. The thickness of the films was obtained using the spectroscopic method. X ray diffraction was performed using Cu K a radiation on a Rigaku Rotaflex Diffractometer. Secondary ion mass spectra (SIMS) depth profiles were obtained on a CAMECA IMS5F instrument.
3. Results and discussion 3.1. Structural properties The X-ray diffractograms of SnO2 films grown (using SnCl2) at substrate temperatures of 300, 350, 400 and 450 jC are shown in Fig. 2(a – d). Films
160 P. Rajaram et al. / Materials Letters 54 (2002) 158–163
Fig. 2. X-ray diffractograms for films grown at substrate temperatures (a) 300 jC, (b) 350 jC, (c) 400 jC, (d) 450 jC.
P. Rajaram et al. / Materials Letters 54 (2002) 158–163
grown at and below 300 jC were amorphous, whereas those grown at 350 jC had a better crystallinity, which improved further as the temperature was increased from 350 to 400 jC. The best crystallinity was obtained at 400 jC, a substrate temperature in which a highly oriented (200) growth occurs. Interestingly, as the substrate temperature is further increased from 400 to 450 jC, the preferred orientation changes from (200) to (211). The crystal structure in all the cases was, however, the tetragonal cassiterate structure. SnO2 films, prepared by the same technique using SnCl4 instead of SnCl2, showed similar results. These two kinds of films, however, differed in the stability of the electrical properties. Although both types of films had high transmission and good conductivity, the electrical properties of the films grown from SnCl4 were unstable to postdeposition annealing. Heat treatments even at temperatures as low as 200 jC (and higher) led to a change in the conductivity of the
161
films. The electrical parameters of the films grown from SnCl2 were, on the other hand, highly stable and did not deteriorate during postdeposition annealing even at temperatures as high as 450 jC. This was particularly important when the SnO2 layers were used in our lab for CdS-based solar cells. Since the CdS was deposited on SnO2 at temperatures around 350 jC, the fact that the sheet resistance of SnO2 did not change during the deposition of CdS was a plus point. Structurally too, the films grown from SnCl2 had a superior crystallinity. 3.2. SIMS analysis SIMS depth profiles of the SnO2 films were obtained to study the uniformity of growth of the layers. Fig. 3 shows the depth profile of the 120Sn and 16 O signals obtained from a typical SnO2 sample on which another semiconductor CuInSe2 was grown. The almost-flat profile along the depth confirms that the composition of the film is uniform along the depth of the film. Depth profiles of heterojunctions like SnO2 /CdS/CuInS2 were also studied and these too confirmed the uniformity of growth of SnO2.
4. Optical and electrical properties
Fig. 3. SIMS depth profile for a CVD grown SnO2 film.
Fig. 4 shows the transmission spectrum of a SnO2 film grown on quartz at 400 jC. It shows that the transmission coefficient is between 94% and 99.9% in the whole visible range. This sample has a sheet resistance slightly less then 100 V/5 and a thickness ˚ . Fig. 5 shows the plot of (a ht)2 vs. close to 2500 A ht whose linear intercept gives a direct band gap of 4 eV, which is close to the values reported in Ref. [9]. ˚ to 1 Layers of higher thickness (between 5000 A Am) have a much lower sheet resistance at the expense of a slight fall in the transmission. Films having a sheet resistance of 10 V/5 with a transmission of about 70% have been routinely obtained with very good repeatability. Further improvement in transmission along with low sheet resistance will require doping of the films with dopants like Fluorine, and such attempts are going on in our lab. The resistivity of the SnO2 films decreased as the growth temperature increased from 300 to 400 jC. Films grown between
162
P. Rajaram et al. / Materials Letters 54 (2002) 158–163
Fig. 4. Transmission curve for SnO2 film grows on quartz.
400 and 450 jC had the lowest resistivity and highest transmission. The best films had a figure of merit defined by T 10/ R 5 [18] around 3.5 10 3. The SnO2 films grown at 400 jC had the lowest sheet resistance, ranging from 10 to 100 V /5, depending on the thickness of the layers. Films with lower sheet resistance 3 – 6 V /5 could be grown by increasing the thickness of the deposit; however, the optical transmission fell down to about 50%.
paper can be used to obtain highly conducting and highly transparent films of SnO2. The method gives fairly uniform SnO2 films and has very good repeatability. Films grown at 400 jC had the best crystallinity with a highly oriented growth. The electrical resistivity of the films grown using SnCl2 are highly stable to postdeposition annealing.
Acknowledgements 5. Conclusions In conclusion, we have demonstrated that the simple and low-cost CVD technique described in this
We are grateful to the Department of Science and Technology (DST), New Delhi, India, for supporting this work. We are thankful to IUC, Indore and IGCAR, Kalpakkam for providing us with the X-ray
P. Rajaram et al. / Materials Letters 54 (2002) 158–163
163
References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] Fig. 5. (a ht)2 vs. ht for SnO2 film (on quartz).
[15]
diffraction and SIMS facilities, respectively. We are also thankful to Dr. S.K. Gupta, S.O.S. in Chemistry, Jiwaji University for providing the spectrophotometer facility.
[16] [17] [18]
J.C. Anderson, J. Vac. Sci. Technol., A 4 (3) (1986) 610. K.D. Wise, J. Vac. Sci. Technol., A 4 (3) (1986) 617. S.M. Budd, Thin Solid Films 77 (1986) 13. A.K. Saxena, S.P. Singh, O.P. Agnihotri, Solar Cells 19 (1986 – 1987) 163. K.L. Chopra, S. Major, D.K. Pandya, Thin Solid Films 102 (1983) 1. H. Nishimura, Bus. Jpn. 9 (1991) 51. P. Smith, R. Singh, J. Dubow, Appl. Phys. Lett. 51 (4) (1980) 2164. K.B. Sundram, G.K. Bhagavat, Thin Solid Films 78 (1981) 35. J.C. Manifacier, M. De Murcia, J.P. Fillard, E. Vacario, Thin Solid Films 41 (1977) 127. G. Gasparro, J. Putz, Sol. Energy Mater. Sol. Cells 38 (1998) 287. S. Tamura, T. Ishida, H. Mihara, O. Tabata, T. Tatsuta, Thin Solid Films 343 – 344 (1999) 142. T. Arai, J. Phys. Soc. Jpn. 15 (1960) 916. S.A. Nasser, Thin Solid Films 342 (1999) 47. A. Bhardwaj, B.K. Gupta, A. Raza, A.K. Sharma, O.P. Agnihotri, Solar Cells 5 (1981 – 1982) 39. A. Ortiz, S. Lopez, C. Falcony, M.H. Farias, G. Soto, L. CotaRaiza, Sol. Energy Mater. 20 (1990) 381. M. Kojima, H. Kato, A. Imai, J. Appl. Phys. 64 (4) (1988) 1902. Y.P. Yadava, G. Denicolo, A.C. Arias, L.S. Roman, L.A. Hummelgen, Mater. Chem. Phys. 48 (1997) 263 – 267. G. Haacke, J. Appl. Phys. 47 (1976) 4086.